Field sequential color display systems which incorporate electro-optic shutters or color switches heretofore have combined the polarization switching capabilities of a twisted nematic liquid crystal electro-optical cell with anisotropic optical properties of separate passive optical elements in an attempt to provide a display with an acceptable color contrast ratio. The transmissive twisted nematic liquid crystal cell is typically included as a two-color switch in an assembly of optical components in which the cell is positioned between two crossed red and green polarizers and a neutral analyzing polarizer near the viewer. If the absorption axis of the analyzing polarizer is aligned with the absorption axis of the red polarizer, the liquid crystal in the absence of an electric field twists through 90.degree. the plane in which the polarized light vibrates so that only a red image passes through the analyzing polarizer. In the presence of an electric field, the liquid crystal cell is turned "ON" and passes the polarized light without rotation so that a green image appears on the analyzing screen.
Display devices incorporating twisted nematic crystal cells that have possible application in sequential color display systems are disclosed in U.S. Pat. Nos. 4,019,808 and 4,239,349 of Scheffer. Color display systems incorporating a twisted nematic liquid crystal cell are disclosed in U.S. Pat. No. 4,003,081 of Hilsum, et al. and U.S. Pat. No. 4,295,093 of Middleton.
It is well known that the twisted nematic cell possesses a relatively slow turn-off time and, therefore, is unacceptable for applications such as a color switch in frame synchronized field sequential color television type display systems. Such systems require the use of a color switch which is capable of responding to signals from synchronization circuits which operate at frame rates of relatively high speed to provide a flicker-free image on the display. To overcome this disadvantage inherent in the twisted nematic cell, cells have been fabricated which include liquid crystal materials characterized as having a dielectric anisotropy that changes from a positive to a negative value as a function of the frequency of a switching signal which is applied to the cell.
A cell which includes liquid crystal material of this type is disclosed in the publication by Raynes and Shanks: "Fast-Switching Twisted Nematic Electro-Optical Shutter and Color Filter," Electronics Letters, Vol. 10, No. 7, pp. 114-115, Apr. 4, 1974. The cell described therein includes a liquid crystal material having a variable dielectric anisotropy mixture which is positive in an electric field produced by a low frequency signal and negative in an electric field produced by a relatively high frequency signal. Thus, the application of a low frequency signal to a twisted nematic cell of this type turns the device "ON," and a high frequency signal pulse forcibly returns the cell to its twisted "OFF" state.
The two-frequency twisted nematic device suffers from the disadvantage of requiring the use of a complex driver which is capable of delivering high frequency signal pulses at high voltage levels to the capacitive load presented by the liquid crystal cell. In addition, it is difficult to make such cells which are capable of uniform switching over large areas and which do not present on a display an image having a patchy appearance. Two-frequency materials also suffer from an inability to operate outside a limited temperature range.
Another optical effect which has been used in liquid crystal display applications is tunable birefringence. A commonly used device of this type is described in the publication "Transient Behavior of Twisted Nematic Liquid-Crystal Layer in an Electric Field," Journal De Physique, Vol. 36, pp. Cl-261-Cl-263 by C. F. Van Doorn. The liquid crystal cell described in the publication by Van Doorn has alignment directors which form tilt bias angles of the same rotational sense as measured from the surface of the cell electrodes. Such a cell, however, experiences "optical bounce" and a consequent protracted relaxation time which render the cell unusable in applications that require short transition times between switching states. The Van Doorn publication states that liquid crystal material flow within the relaxing cell is responsible for the occurrence of the optical bounce phenomenum. The direction of liquid crystal material flow within the cell appears to apply a reverse torque to the local directors centrally located within the cell, which torque is in opposition to the direction of local director realignment during relaxation of the cell and thereby causes optical bounce and increased relaxation times.
A variable retardation device including a liquid crystal cell of substantial thickness which eliminates optical bounce and thereby possesses short relaxation times between optical states is reported in a publication "Performance of a Matrix Display Using Surface Mode," 1980 Biennial Display Research Conference Proceedings, pp. 177-179 by James L. Fergason. The device described in the publication by Fergason is unacceptable in most image display applications, however, because of its severely restricted cone of view, which is inherent in liquid crystal cells of substantial thickness.
A device using the properties of birefringence in materials other than liquid crystals is disclosed in U.S. Pat. No. 2,638,816 of Stolzer which describes an adapter to generate color images from a black and white television set. The adapter of Stolzer includes a cell which experiences the Kerr effect, which is the designation of the characteristic of certain isotropic substances that become doubly refractive in the presence of an electric field.
The adapter of Stolzer polarizes light emitted from the television set. The Kerr cell receives the polarized light and separates it into orthogonally related components. The amount of retardation of one of these components relative to the other varies as the function of the electric field strength produced by an external voltage which is applied to the cell electrodes and which varies in synchronism with the frame sequential operation of the television set. The light is then passed through passive birefringent sheets to produce a light output in different colors. One disadvantage inherent in the device of Stolzer is that colors developed from passing light through passive birefringent sheets generally are impure and vary in appearance as a function of the viewing angle. The adapter also employs interdigital electrodes which present a pattern of lines across the display screen. Thus, the device of Stolzer produces color images which are not acceptable for most image display applications.